Protonation of CO2, COS, CS2: Proton affinities and the structures of the protonated species

Abstract

Abstract The geometric structures of a number of isomers of the ions formed by protonation of CO 2 , COS and CS 2 , and of the parent molecules themselves, have been fully optimized using ab initio quantum chemical methods. Stable minima have been found both for molecules protonated at the terminal atom and at the central carbon atom; ions of the latter type show strong near-degeneracy effects which have been ignored in previous calculations. Proton affinities of CO 2 , COS and CS 2 have been calculated: for CO 2 the theoretical result (565 kJ mol −1 ) is in excellent agreement with experiment (540 kJ mol −1 ), given that the experimental proton affinity includes a contribution from zero-point vibration of ≈ −27 kJ mol −1 . For COS, for which no experimental value is available, the calculations give almost identical results for both O and S protonated species (619 and 636 kJ mol −1 , respectively). It may not therefore be possible to distinguish these two isomers experimentally. The theoretical result for CS 2 (678 kJ mol − ) suggests that the current experimental value of the proton affinity (699 kJ mol −1 ) is too high, since this value includes a zero-point vibration contribution of some −19 kJ mol −1 ).